Platelets are very small cellular components of the blood that comprise an outer membrane and a cytoplasm containing granules, dense bodies, dense tubular system and mitochondria. Platelets are produced in the bone marrow and survive in the circulatory system for an average of 9 to 10 days before being removed from the body by the spleen. The main function of platelets is the maintenance of hemostasis, the body's normal physiological process that prevents excessive blood loss. Hemostasis is primarily achieved by the formation of a thrombus (or blood clot) following damage to the endothelium of blood vessels.
Platelet transfusions are used extensively in the medical field, in particular for assisting in the control of bleeding and for replacing functionally defective platelets in patients. For example, platelet transfusions are administered to trauma patients who have lost significant amounts of blood, to patients undergoing chemotherapy, which is known to reduce the number of platelets and cause functional defects in remaining platelets, as well as to patients with certain platelet-depleting diseases (e.g., thrombocytopenia).
Platelets used for transfusion can come from two sources: platelet concentrates derived from units of whole blood, termed random donor platelet concentrates, or apheresis platelets obtained from a single donor by plateletpheresis, a technique which involves continuous separation of platelets from a donor, with simultaneous reinfusion of blood minus platelets back into the donor.
Whatever the mode of collection or use, platelet preservation presents problems that are not observed with the storage of whole blood or other blood components. Indeed, it has proved difficult to preserve platelets after their isolation from the body under conditions not only maintaining the biological activity of platelets but also suitable for clinical use (e.g., absence of bacterial contamination). Refrigeration of platelets is associated with several limitations. Indeed, at 4° C., platelets aggregate and undergo a morphological change after one day of storage which results in biological dysfunction. Consequently, platelets stored at 4° C. generally fail to recover functional activity and often undergo lesions with storage that cause them to be removed from the circulation following transfusion. Therefore, the standard means of storage of platelets is at room temperature (18-24° C.) with gentle agitation. However, even under these conditions, storage duration is limited to 3 to 5 days, because of a decrease in pH due to increased lactate production associated with anaerobic metabolic activity.
Since under standard storage conditions, platelets have a shelf-life of only a few days, there has been considerable interest in devising new strategies for extending storage duration and for diminishing or delaying the loss of platelet function during the storage period (Read et al., Mol. Med. Today, 1995, 1: 322-328). One of the different approaches developed makes use of cryopreservation techniques. These methods generally provide an increased number of platelets following storage. However, freezing temperatures require the addition of cryoprotectors, such as dimethyl sulfoxide, to platelets. Cryoprotectors are cytotoxic and typically leave a significant portion of the platelets with either reduced or no functional ability (Blajchman, Transf. Clin. Biol., 2001, 8: 267-271).
Other attempts to preserve platelets have included the use of platelet activation inhibitors (Bode et al., Blood Cells, 1992, 18: 361-380), of plasma-free platelet storage media (de Wildt-Eggen et al., Vax Sang, 2003, 84: 256-264), and of platelet additive solutions (Gulliksson, Transfus. Med., 2000, 10: 257-264; van der Meer, Transfus. Clin. Biol., 2007, 14: 522-525) as well as the development of platelet products (George et al., Blood, 1982, 60: 834-840; McGill et al., J. Lab. Clin. Med., 1987, 109: 127-133) and of platelet substitutes (Okamura et al., Bioconj. Chem., 2009, 20: 1958-1965; Okamura et al., Bioconj. Chem., 2005, 16: 1589-1596).
Another avenue of research for the preservation of platelets has focussed on the use of lyophilization (Aster, Proc. Natl. Acad. Sci. USA, 1995, 28: 2419-2420; Read et al., Proc. Natl. Acad. Sci. USA, 1995, 92: 397-401; Bode et al., Transf. Sci., 2000, 22: 99-105; Bode et al., Art. Cells Blood Substit. Immobil. Biotechnol., 2007, 35: 125-133). However, this technique has not yet provided satisfactory results since, even when storage is performed in the presence of protective glycoproteins, platelets preserved by freeze-drying, do not circulate in the body after transfusion.
Therefore, there is a great need for improved methods for the preservation of blood platelets suitable for transfusion. In particular, preservation methods that would allow platelets to be stored for long periods of time while maintaining their viability and bioactivity and reducing the likelihood of bacterial growth are highly desirable.